Fibroblast Growth Factors and their Receptors
Fibroblast growth factors (FGFs) comprise a large family of evolutionarily conserved polypeptides involved in a variety of biological processes including morphogenesis, angiogenesis, and tissue remodeling as well as in the pathogenesis of numerous diseases (reviewed in Ornitz, Bioessays 22, 108, 2000). The various members of this family stimulate the proliferation of a wide spectrum of cells, ranging from mesenchymal to epithelial and neuroectodermal origin in vitro and in vivo. FGFs are expressed in a strict temporal and spatial pattern during development and have important roles in patterning and limb formation (reviewed in Ornitz, Bioessays 22, 108, 2000).
FGFs are powerful mitogens and are critical in the regulation of many biological processes including angiogenesis, vasculogenesis, wound healing, limb formation, tumorigenesis and cell survival. The biological response of cells to FGF is mediated through specific, high affinity (Kd 20-500 pM) cell surface receptors that possess intrinsic tyrosine kinase activity and are phosphorylated upon binding of FGF (Coughlin et al. J Biol. Chem. 263, 988, 1988). Five distinct Fibroblast Growth Factor Receptors (FGFRs) have been identified, FGFR1-4 are transmembrane-protein kinases while FGFR5 appears to be a soluble receptor. The FGFR extracellular domain consists of three immunoglobulin-like (Ig-like) domains (D1, D2 and D3), a heparin binding domain and an acidic box. Alternative splicing of the FGFR mRNAs generates different receptor variants, including the FGFR3IIIb and FGFR3IIIc forms, each having unique ligand specificity.
Another critically functional component in receptor activation is the binding to proteoglycans such as heparan sulfate. FGFs fail to bind and activate FGF receptors in cells deprived of endogenous heparan sulfate. Different models have been proposed to explain the role of heparan sulfate proteoglycans (HSPG) in FGF signaling, including the formation of a functional tertiary complex between FGF, FGFR and an HSPG (Yayon et al., Cell 64, 841, 1991; Faham et al, Curr. Opin. Struct. Biol. 8: 578, 1998).
Bone Development
The process of bone formation is initiated by endochondral ossification and intramembranous ossification. Endochondral ossification is the fundamental mechanism for longitudinal bone formation whereby cartilage is replaced by bone. It requires the sequential formation and degradation of cartilaginous structures in the growth plates that serve as templates for the developing bones. During intramembranous ossification, bone is formed directly in the connective tissues. Both processes require the infiltration of osteoblasts and subsequent matrix deposition.
The signaling pathway triggered by activation of FGFRs has been shown to be involved in several stages of limb and bone development. Other major regulators of bone growth include natriuretic peptides (NP), bone morphogenetic proteins (BMP), growth hormone (GH), insulin-like growth factors (IGF), glucocorticoids (GC), thyroid hormone (TH), parathyroid hormone (PTH), PTH related peptide (PTHrP) and Vitamin D.
FGFRs and Disease
A number of birth defects affecting the skeleton are associated with mutations in the genes encoding FGF receptors, specifically Crouzon, Pfeiffer, Jackson-Weiss, Apert and Beare-Stevenson syndromes (Kan, et al., Am J Hum Genet. 70, 472, 2002). Mutations in FGFR3 are responsible for achondroplasia, the most common form of human genetic dwarfism (reviewed in Vajo et al., Endocr. Rev. 21, 23, 2000). Specifically, the outcome of the achondroplasia mutation is a stabilized, constitutively activated FGFR3 leading to restricted chondrocyte maturation in the growth plate of long bones and abnormally shortened bones.
The FGFRs have been implicated in certain malignancies and proliferative diseases. FGFR3 is the most frequently mutated oncogene in transitional cell carcinoma (TCC) of the bladder where it is mutated in more than 30% of the cases (Cappellen et al., Nature Genet. 23, 18, 1999). Dvorakova et al. (Br. J. Haematol. 113, 832, 2001) have shown that the FGFR3IIIc isoform is over expressed in the white blood cells of chronic myeloid leukemia (CML) patients. Yee et al. (J. Natl. Cancer 92, 1848, 2000) identified a mutation in FGFR3 linked to cervical carcinoma. Recently, FGFR4 was shown to be associated with pituitary tumors (Ezzat, et al, J. Clin. Invest. 109, 69, 2002) and breast cancer progression (Bange, et al., Cancer Res. 62, 840, 2002).
In contrast, FGFs and their analogs have been shown to be useful for treating indications including wounds (U.S. Pat. Nos. 4,950,483, 5,859,208 and 6,294,359), myocardial infarction (U.S. Pat. Nos. 4,296,100 and 4,378,347), skeletal disorders (U.S. Pat. Nos. 5,614,496 and 5,656,598) and for remodeling cardiac tissue (U.S. Pat. No. 6,352,971).
Receptor Specificity
In light of the large number of FGFs and FGF receptor variants, a major question regarding FGF function is their receptor specificity. All FGFRs tested so far bind FGF-1 (acidic FGF, aFGF) with moderate to high affinity, demonstrating an apparent redundancy in the FGF system. In contrast to FGFR1 and FGFR2, the third receptor subtype, FGFR3 was found to bind to FGF-8, FGF-17 and FGF-18 with high affinity and to FGF-9 with improved selectivity. Specificity may also be achieved by specific proteoglycans expressed in different tissues (Ornitz, Bioessays, 22, 108, 2000). Site-directed mutagenesis and X-ray crystallography were used to study the basis of specificity of FGFs to their receptors (Plotnikov et al., Cell 98, 641, 1999; Plotnikov et al., Cell 101, 413, 2000; Stauber et al., PNAS 97, 49, 2000; Pellegrini et al., Nature, 407, 1029, 2000; Schlessinger et al., Mol Cell, 6, 43, 2000).
FGF Variants
All members of the FGF family share a homology core domain of about 120 amino acids, 28 aa residues are highly conserved and six are identical. Structural studies on several FGFs identified 12 antiparallel β strands each one adjacent to β-loops comprising the core region, conserved throughout the family. The core domain comprises the primary FGFR and heparin binding sites. Receptor binding regions are distinct from heparin binding regions (reviewed in Ornitz and Itoh, Gen. Biol. 2, 3005.1, 2001).
Attempts have been made to achieve altered FGF receptor specificity by deletions or truncations of its ligands, by means of mutations introduced at certain locations within the gene encoding for the proteins. Copending PCT application WO 02/36732 discloses FGF variants having at least one mutation in the β8-β9 loop, having increased receptor specificity to one receptor subtype compared to the corresponding wild type FGF.
Several investigators have demonstrated FGF mutants and variants affecting receptor and heparin binding. Kuroda et al., (Bone, 25, 431, 1999) demonstrated that a full-length FGF-4 polypeptide and a shortened version containing 134 amino acid residues exhibit comparable cellular proliferation and effect on increase of bone density. The shortest form of FGF-4 tested, containing only 111 amino acid residues, exhibited limited growth stimulatory activity.
U.S. Pat. No. 5,998,170 discloses a biologically active FGF-16 molecule having from one to thirty-four amino acids deleted from the N-terminus or from one to eighteen amino acids deleted from the C-terminus.
U.S. Pat. No. 5,512,460 discloses an FGF-9 (glia activating factor, GAF) molecule comprising N-terminus and C-terminus truncations of 53 aa and 13 aa, respectively. U.S. Pat. No. 5,571,895 discloses a 54 aa deletion from the N-terminus of the protein yielding a 154 aa protein retaining its biological activity.
Basic FGF, also known as FGF-2, bFGF, prostatin and heparin binding growth factor 2, is highly conserved among species and has been shown to stimulate the proliferation of a wide variety of cell types. The sequence of FGF-2 has been disclosed U.S. Pat. Nos. 4,994,559; 5,155,214; 5,439,818 and 5,604,293. Human FGF-2 is expressed in several forms, a 210 aa precursor, a 155 aa form, a 146 aa N-terminal truncated form and several others (reviewed in Okada-Ban et al., Int J Biochem Cell Biol, 32, 263, 2000).
FGF-2 has been modified to alter biological properties and binding specificity. U.S. Pat. No. 5,491,220 discloses structural analogues comprising substitution of the β9-β10 loop having altered biological properties and binding specificity. Seno et al. (Eur. J. Biochem. 188, 239, 1990) demonstrated that removal of the C-terminus, not the N-terminus, affects FGF-2 affinity to heparin.
Bailly et al. (FASEB J, 14, 333, 2000) show that FGF-2 mitogenic and differentiation activities may be dissociated by a point mutation in Ser117 (S117A).
Human FGF-2 superagonists have been designed with substitutions at either one or more of the following amino acids: glutamate 89, aspartate 101 and/or leucine 137 (U.S. Pat. No. 6,274,712; note that the aa numbering is according to the 146 aa form of FGF-2 disclosed in Zhang et al, PNAS 88: 3446, 1991). U.S. Pat. No. 6,294,359 discloses agonist and antagonist analogs of FGF-2 that comprise amino acid substitutions at certain heparin and receptor binding domains but does not teach receptor specificity changes.
U.S. Pat. Nos. 5,302,702 and 5,310,883 disclose a recombinant FGF-2 variant, having the alanine of position 3 and the serine of position 5 replaced with glutamic acid, exhibiting increased yields.
The use of FGFs and FGF fragments for targeting cytotoxic agents has been disclosed in WO 01/39788 and U.S. Pat. Nos. 5,191,067; 5,576,288; 5,679,637. A mitogenically active FGF molecule provides a route for introducing the selected agents into the cell.
The extensive efforts made to produce truncation, deletion and point mutation variants in FGF have resulted in changes in affinity to the receptors but not in significant alterations in receptor specificity. Thus, there is an unmet need for highly active and selective ligands for the various types of FGF receptors, useful in selective stimulation or inhibition of these receptors, thereby addressing the clinical manifestations associated with the above-mentioned mutations, and modulating various biological functions.
It is to be explicitly understood that known variants of FGFs are excluded from the present invention.